The controversy even led Paris Jackson, the late pop star's 14-year-old daughter, to use her Twitter account to weigh in.

"i am going to clarify right now that what has been said about my grandmother is a rumor and nothing has happened, she is completely fine," Paris tweeted.

Relations between the Jackson family and executors John Branca and John McClain have been strained for most of the three years since Michael Jackson's sudden death from an overdose of an anesthetic used as a sleep aid.

The letter mostly repeats allegations made by family members, but it also threatens new legal action against Branca, an entertainment lawyer, and McClain, a music executive, who worked with Jackson at several points in his career.

"We know there is most certainly a conspiracy surrounding our brother's death and now coarse manipulation and fear are being used to cover it up," they wrote. "Your heartless pursuit of wealth, fame and power is at the expense of our family, whose deepest desire is to give to the world a gift of hope, love and unity through our music."

It was signed by Janet, Jermaine, Tito, Randy and Rebbie Jackson. Jackie, Marlon and La Toya Jackson did not sign it, nor did parents Joe and Katherine Jackson.

"We are saddened that false and defamatory accusations grounded in stale Internet conspiracy theories are now being made by certain members of Michael's family whom he chose to leave out of his will," estate spokesman Jim Bates said in a statement to CNN. "We are especially disheartened that they come at a time when remarkable progress has been made to secure the financial future of his children by turning around the estate's finances as well as during a time when so many of Michael's fans, old and new, are enjoying his artistry through exciting new projects."

The will, which was validated in probate court in the months after Jackson's June 25, 2009, death, put all Jackson's assets into a trust that benefited his mother, Katherine; his children, Prince, Paris and Blanket; and charities. Branca and McClain were named as executors.

The signature page indicated that Michael Jackson signed the will in Los Angeles on July 7, 2002, on which date the Jacksons say he was in New York.

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"According to what is witnessed in the document, it is impossible and illogical that he could have been in two places at one time," the letter said. "We have evidence that undoubtedly supports and proves that Michael was absolutely not in Los Angeles, California, on the date his signature reflected in the will at hand."

They also questioned why their brother would name as executors someone he disliked.

"Our brother told us, in no uncertain terms and without hesitation in the months prior to his death, that he despised both of you and that he did not want either of you to have anything to do with his life or estate for that matter," the letter said. "We know that and you know that."

Courts have already settled the matter, the estate spokesman said.

"Any doubts about the validity of Michael's will and his selection of executors were thoroughly and completely debunked two years ago when a challenge was rejected by the Los Angeles County Superior Court, the California Court of Appeals and, finally, the California Supreme Court," Bates said.

The Jackson brothers and sisters also accuse Branca and McClain of abusing their 82-year-old mother through lies and manipulations.

"Your actions are affecting her health, and on top of that, we've just found out she recently had a mini-stroke. Please understand, she's not equipped to handle the stress load you are putting on her," her children said. "She feels, as she has said, 'I'm stuck in the middle.' She too knows and acknowledges the will was forged. She wants to do the right thing, and move in the direction of justice for her son and family, yet she fears the POWERS THAT BE."

Katherine Jackson's lawyer Perry Sanders disputed that she was in poor health.

"Mrs. Jackson is extremely lucid and does a great job caring for Michael's children," Sanders said Wednesday.

The siblings also attacked Sanders, Katherine Jackson's manager, Lowell Henry, and adviser Trent Jackson for discouraging her from joining their challenge of the will's validity.

"Instead, her so-called advisers are convincing her to let them negotiate 'deals' with Branca and McClain on her behalf, or is it on the behalf of all of you," they wrote. "Her advisers' loyalty seems to be skewed by the percentage you offer them, preventing them from advising her properly."

Sanders responded that he's always been available "to speak with or meet with any of Mrs. Jackson's children to answer any questions they might have. It is unfortunate that they haven't taken me up on the offer."

"Anyone who actually knows me knows my only loyalty in this matter is to Mrs. Jackson, and the public record of my success for her speaks for itself," Sanders said. He pointed to success in paying off her $14 million civil court judgment to a South Korean company, persuading the estate not to sell the family's Hayvenhurst mansion and "helping get her family allowance increased almost tenfold, to name a few."

"Your greed and hasty business decisions have shown that you have no regard for the preservation of his legacy, nor the quality of work that he exemplified," they said. "You have disrespected our parents and family too many times. We do not respect you as executors, and we don't respect the projects and choices you've made, nor do we appreciate the public perception that the Jackson family is behind all of this, exploiting Michael our brother for financial gain, when it's the two of you and your affiliates who do so."

Finally, the five Jacksons wrote that they would "take every appropriate action to seek justice and to see to it that the truth be known."

"Be informed, we are considering retaining a law firm, Baker Hostetler, who have advised us on the potential criminal misconduct in your actions. We will hand this over to proper authorities," they wrote.

Bates said the executors "have diligently carried out their fiduciary duties as well as their obligation to Michael to make sure that his estate benefits the only family members he named in his will, his mother and his three children."